The ethnic minorities from the north and the centre of Laos practice slash and burn agriculture. This method seems to be particularly aggressive to the environment. In a cycle of six to ten years, patches of forest are burned down to plant rice, corn, and opium since the last 200 years. The impact on the environment is in fact not irreversible, as long as the demographic pressure does not shorten this cycle of six to ten years, giving the forest time to regenerate. The Hmong have an extremely rich animist religion, and they have successfully been resisting outside influences on their culture for the last fifty years.